Reliable optical communication systems require mechanisms for minimizing the effects of signal degradation occurring between associated transmitters and receivers. Signal degradation occurs due to a variety of factors that cannot be completely eliminated, and is exacerbated by the long-haul transmission distances and high optical channel counts required in many applications. Due to signal degradation, some transmitted data may be incorrectly interpreted at a receiver. If data is misinterpreted at a rate above that which is acceptable, the efficacy and viability of the system may be lost.
A variety of techniques for minimizing the effects of signal degradation have been investigated. Forward Error Correction (FEC) is one technique used to help compensate for signal degradation and provide “margin improvements” to the system. Margin improvements generally allow an increase in amplifier spacing and/or increase in system capacity. In a Wavelength Division Multiplexing (WDM) system, for example, margin improvements obtained through FEC techniques allow an increase in the bit rate of each WDM channel and/or a decrease the spacing between WDM channels. This translates directly into an increased system data capacity.
FEC typically involves insertion of a suitable error correction code into a transmitted data stream to facilitate detection and correction of data errors about which there is no previously known information. Error correction codes are generated in an FEC encoder for the data stream, and are sent to a receiver including an FEC decoder. The FEC decoder recovers the error correction codes and uses them to correct any errors in the received data stream.
There are a large number of error-correction codes, each with different properties that relate to how the codes are generated and consequently how they perform. Some examples of these are the linear and cyclic Hamming codes, the cyclic Bose-Chaudhuri-Hocquenghem (BCH) codes, the convolutional (Viterbi) codes, the cyclic Golay and Fire codes, Reed-Solomon (RS) codes, and some newer codes such as the Turbo convolutional and product codes (TCC, TPC).
Of course, the efficacy of FEC techniques is impacted by the ability of the optical signal receiver to correctly detect transmitted data and error correction codes. Improvements in receiver signal detection thus translate to improved performance of FEC codes in providing correction of bit errors. Accordingly, there is a need for an optical receiver configuration that provides accurate and reliable detection of the received data signal to provide improved error correction results.